The present invention relates generally to molds required to cure large radial pneumatic tires.
It is well known that conventional radial tires run cooler and therefore provide longer service hours than conventional bias-ply tires. The superiority of a conventional radial tire, as compared to a bias-ply tire, results from the provision in a radial tire of substantially inextensible cables arranged in generally radial planes of the tire from one bead to the other bead, with a circumferential belt package of low angle cords (e.g., 5-7 degrees relative to a radial plane of the tire) being secured radially outwardly of such cables. The provision of the cables and the belt package restrains circumferential expansion of a radial tire, and accordingly, the belt package cords do not scissor relative to one another. The radial tire therefore operates at a lower temperature. The conventional bias-ply tire does not utilize a circumferential belt package and instead employs a plurality of cris-crossed plies which extend at relatively large angles relative to a radial plane of the tire. This construction subjects the tire to a heat build-up generated by the tendency of the plies to undergo scissoring relative to one another, and it is well known that excess heat tends to cause delamination and rapid tread wear resulting in a reduced service life.
Tire molds for curing complete tires are generally of two types: full circle molds and segmental molds. In the case of segmented molds, heated exterior components of the mold are moved into contact position with the tread and sidewall portions of the uncured tire and cure the tire from the outside, while a bladder is inflated to contact the inner surface of the tire to help shape the tire, and heating fluids are injected into the inside of the bladder to cure the tire from the inside. An example of a full circle clamshell mold is shown in U.S. Pat. No. 4,957,676 to Greenwood. Full circle clamshell molds are proven, reliable, and lower in cost than segmental molds to manufacture. Another benefit of conventional clamshell molds is that a significant portion of the mold, typically the bottom half, may remain stationary as only the top half needs to be raised and lowered with respect thereto by a molding press, along an axis of rotation of the tire, to open and close the mold. However, clamshell molds are not appropriate for molding radial tires because a radial tire will not expand or contract in diameter and the inner molding surface of the clamshell mold cannot move radially outwardly to receive an uncured tire and radially inwardly to close around a tire casing.
Another type of mold for curing tires is the segmental mold, examples of which are shown in U.S. Pat. No. 5,676,980, to Gulka et al. U.S. Pat. No. 3,787,155 to Zangl, and U.S. Pat. No. 3,806,288 to Materick. Unlike clamshell molds, which are split about the centerline of the tread pattern, segmental molds are radially segmented into a plurality of arcuate tread segments about the circumference of the mold. Each of the segments is attached to a top section of a mold container so that when the top section is lowered and raised by a press in which the container is installed, the mold section correspondingly moves up and down along the axis of tire rotation. In this direction, movement of the top mold section corresponds generally to that of the top portion of a clamshell mold. The fact that a radial tire is not radially expandable or contractible creates serious manufacturing problems when attempting to mold an uncured radial tire in a conventional segmental mold. Segmented molds such as shown in Gulka U.S. Pat. No. 5,676,980, are indicated as being openable to the full outside diameter of a radial tire, but present a serious problem to curing a satisfactory large diameter radial tire (e.g., about 50 inches in diameter), particularly a heavy tire for use on large off-the road vehicles, i.e., unless the mold is used in conjunction with a molding press that maintains the uncured tire in a centered position as the mold closes When the mold is opened to receive a heavy uncured tire there exists a vertical and horizontal gap between the radially outer edge of the lower mold sections"" sidewall plate and the radially inner edge of the lower tread segments. Accordingly, as the uncured tire is lowered into the cavity of the lower mold section, the heavy weight of the tire causes the cords of the belt package tire to flex which in turn causes the tire""s belt package to be deformed relative to the uncured rubber in which the cords are encased as they encounter the aforementioned gap. Because of such deformation, discontinuities between the belt package cords and the uncured rubber are created, precluding the formation of an integral bond between the cords and the rubber when the tire is cured. As a result, there is a likelihood that delamination of the belt package cords relative to the tire body will occur seriously reducing the service life of the tire. The deformation can be even severe enough to render the cured tire unusable.
The present invention provides an improved method and apparatus for curing a radial tire within a segmental mold in a conventional autoclave without deforming the cords of the belt package relative to the uncured ruber of the tire. The segmental mold of the present invention includes a top mold section and a bottom mold section. The top mold section is movable with respect to the bottom mold section between a raised open position and a lower closed position. The top and bottom mold sections each contain a plurality of like tread segments arranged in a circular pattern. Each segment is slidably coupled to its mold section by a slanted alignment pin, and two compression springs are positioned on either side of the alignment pin to bias the treads segments outwardly with respect to their respective mold sections. Each of the tread sections is formed with radially inwardly extending tread groove-defining lugs. The bottom mold section is so configured that when the top mold section is in its open raised position, the tread segments of the bottom mold section are arranged radially outwardly of the outer diameter of the uncured tire to be molded. When the top mold section is moved to its closed lower position, the tread segments of both the top and bottom mold sections are automatically moved radially inwardly so that the tread-defining lugs of the segments engage the uncured crown of the tire during curing of the tire. When the tire has been cured, the top mold section is raised and the upper and lower tread segments automatically move outwardly away from the cured tire so such tire can be freely withdrawn from the bottom mold section without the tread segments tearing the tire.
It is a particular feature of the present invention that an uncured tire can be cured in a mold without any deformation taking place between the cords of the bead package and the uncured rubber of the tire thereby precluding the formation of discontinuities between the cords and the uncured rubber surrounding such cords. This feature is accomplished by extending the lower sidewall of the bottom mold section substantially the entire distance from the tire bead area to the periphery of the tread portion of the uncured tire. With this arrangement, the complete sidewall of the uncured tire is supported by the lower sidewall of the bottom mold section and the latter provides a firm platform for the uncured tire without causing a deformation of the belt package of such tire. Additionally, the radially outer portion of the lower sidewall of the bottom mold section is formed with auxiliary lugs that define the lower outer sidewall portion of the treads of a cured tire. The lugs of the tread segments and the lower mold sidewall auxiliary lugs cooperate to form a complete lower tread pattern of a cured tire. The lugs of the upper mold section tread segments and the upper mold sidewall cooperate to form the upper tread pattern of a cured tire.
It is another feature of the present invention that a plurality of the aforementioned molds each containing an uncured tire may be arranged in a conventional autoclave for concurrent curing of the tires. After the tires have been cured the molds are removed from the autoclave and the cured tires withdrawn from their respective molds.
Yet another feature of the present invention is that a single size mold may be utilized to cure tires of varying width.
Further advantages afforded by the method and apparatus of the present invention will become apparent from the following detailed description, when taken in conjunction with the accompanying drawings.